A digitizer tablet computer has been designed which uses a rechargeable battery as the primary power source allowing a user to carry the computer about from place to place and conduct different transactions. Thus, one of the main objectives of such design was to prolong the useful life of the battery as much as possible by using power management techniques disclosed in the above related application (2), and by accurately measuring the power status of the battery and accurately indicating to the user how much time remains before the battery will be fully depleted. Such indication thereby not only allows the user to save any critical current data but also to use the computer as long as possible before the battery becomes depleted and has to be recharged or replaced with a charged battery.
The digitizer tablet computer was also designed to use a nickel-cadmium (nicad) battery pack as a power source since such type of battery has a number of well known advantages. The battery pack has a plurality of serially connected battery cells which, as the battery packs discharges during operation, provide a voltage that decreases gradually from a fully charged condition until the battery nears depletion whereupon the voltage decreases relatively rapidly. It is desirable to provide the user with information on battery life and allow the user to voluntarily save files and data before the possibility of loss occurs. To this end, a battery gauge is provided which is the subject of the above related application (4). The present invention is directed to accurately monitoring the battery voltage and automatically shutting the computer down when the battery is nearly depleted or when cell polarity reversal is eminent.